In large-scale manufacturing complexes to produce basic or specialty chemicals, numerous materials are provided into a vessel for a desired reaction. Changes in one of the feedstreams affects the flowrates of the other streams. To permit maintenance and reduce plant downtime, pumps are normally redundant so that one serves as a spare while the other is operating to provide one of several flow streams to a reactor vessel, for example. When only a single pump is running, an automatic control system can be tuned to the system dynamics so that it responds cleanly and within the desired timeframe to input changes from a controller. Typically, in large manufacturing operations, the 2-pump arrangement comprises a pair of identical centrifugal pumps whose output and discharge pressure varies in a predetermined manner on the basis of pump speed, as indicated by available pump curves from the pump manufacturer.
Such process pumps can be driven by electric motors or other types of drivers, such as steam turbines.
In many situations, the capacity of manufacturing plants is increased years after they are originally built for a given capacity. As part of such throughput increase or debottlenecking, higher flowrates are required of the constituent components, for example, that would go into a reactor vessel. When these situations arise, one alternative is to simply purchase larger pumps to handle the higher throughput and continue the old way of operating, with one pump running and the other sitting idle as a spare. However, reconfiguring the piping, foundations, utilities and the associated downtime can make such a changeover to larger-capacity pumps economically unattractive. Instead, in many chemical processing plants, the decision has been made to run the main and spare pumps together. These pumps are piped in parallel with the objective being that they share the new and higher throughput rates required. When these pumps are driven by turbines, a common plant practice, and are operated simultaneously, control problems arise in unequal responses to governor valve movements associated with each of the turbine drivers. Thus, to modulate two pumps running simultaneously, each having its own controller for positioning the steam inlet valve to a turbine driver, a situation of pumps fighting each other occurs as a flow correction to the steam flow to one of the turbines changes the output of its associated pump and changes the steam flow requirements to the other turbine connected to a common manifold in order to compensate. The steam valves have their own structural features which could affect their rates of movement, such as friction in the actuation assembly or the valve mechanism itself. This continual correction between the governor valves can eventually result in unstable turbine operation or high vibration measured at one or both of the turbines, which could result in an automatic shutdown. Additionally, the two pumps, when both operating in automatic mode, may not respond well to large changes in the measured variable which call for, for example, a sudden decrease in total pump output from both pumps. In prior systems, a master controller controlling the steam flow to the individual pumps would attempt to rapidly reduce the output of both pumps. Depending on the size of the upset, the drastic reduction in output could result in opening of a minimum flow recycle valve on both of these pumps, resulting in a major loss of forward fluid flowrate and, ultimately, the complete shutdown of the pumps and the processing unit. This, of course, is undesirable.
Thus, one of the objectives of the present invention is to provide a control system for multiple pumps connected in parallel which prevents them from fighting each other during normal deviations from a desired setpoint for a measured variable. On the other hand, another objective is to make the control system able to respond to dramatic changes in the measured variable without a loss of both pumps and, hence, a process unit shutdown.
The objectives of the present invention have been addressed by allowing a master controller to present different output signals to the various pumps in response to a change in the measured variable. As a result, the performance change in response to a change in the measured variable in one pump is different than the other. This solution enables the pumps to run together automatically without fighting and further enables them to respond to dramatic changes in the measured variable. The prior pump control systems which are known do not address this problem. Typical of such prior control systems for pumps or applicable to them are U.S. Pat. Nos. 5,566,709; 5,522,707; 5,360,320; 5,259,731; 3,872,887; 3,775,025; 4,686,086; and 4,428,529.